Its easy to change nick on forum, but to avoid confusion and to prevent problematic members, it is disabled. On request we can easily change any nick and there are 2 ways to do that, 1st is easy to do - changing displayed name, but when you are going to login you have to use firstly registered name, so the real name is still there and visible for admins on your profile with current nick, and for everyone else that looks like original nick. 2nd way is complete change and I'm not touching that much, it require email verification and can cause problem for member, but I think its not hard to do.
Anyway changing nicknames may happen only under special circumstances. For forum is better to not happen.
It would make it a duplicate account and most forums don't like that sort of thing.
It takes 2 nicks like explained above.